You can not reprogram the starter lock part of the key your self, your dealer should program all keys you have again so the one lost is gone from the system

__________________
X5 4,4i-06 Current
330Ci Cab-03 2007-2008
MB E55 AMG-03 2006-2007
Z3 2,8 Roadster-98 2006-2007
X3 2,0d-05 2005-2006
320d-99 1999-2005